Cisco 2950 Full Duplex 2

[SCLDTony]

I have inherited a network that uses Cisco 2950 switches. I *think* I have a grip on how they are set up, but I have a question about the Duplex light on the front of the switch. From what I understand, when this LED is lit, the switch is in full duplex mode. Does this mean every port on the switch is now full duplex? Isn't there a software setting in the Cisco IOS for each ports duplex options?

A more basic question: I understand that full duplex provides faster throughput, but can someone explain to me the pro's and con's of using full duplex mode? Can the ports be set to auto and the nic on the machine connected to that port set to auto? Thanks in advance for any assistance!

Tony

 

[lui3]

Full duplex, in 100 base T environs, means that one pair of wires on the ethernet transceiver is used to transmit on the wire while the other pair is used to receive data. each side of the connection is reversed hence resulting in 100 mbps throughput sending and 100 mbps throughput receiving at the same time resulting 200 mbps aggregate throughput.

full duplex also has the advantage of being able to run without any collision detection in place. in fact the feature is turned off in 100 full duplex environs. this means that the transceivers can send at nearly the speed of the wire or 100 base T almost at the full 100 mbps in both directions.

with half duplex environs collision detect must be turned on since the medium is shared. same pair is used to send and receive so that the transceiver must be aware of collisions and be able to back off when it detects one. this gets very inefficient from a overall throughput perspective as the number of collisions increase. collisions occur when two or more stations try to transmit at the same time resulting in frame collisions and subsequent data corruption. the transceivers back off for a random amount of time and retransmit the damaged data. overall this results in about 40% max efficiency in a best case scenario and that is only 40% of the shared speed be it 100 or 10 meg or other. you see more half duplex in shared hub environs. most switches these days default to full duplex.

yes you can set the switch to auto negotiate the port speed/duplex just like the client. in fact in most cases that is on by default as is probably the case with your switches. it is much easier to deal with from an administrative perspective than having to statically set both clients and switch ports to hard full duplex 100 speeds. so seeing the lights are on the switch means the ports are successfully negotiating the 100 full connections with the clients. this is just a guest. it is possible that the admins have set the ports on clients switch to full on both sides as well. you will have to check

anyways for best results stick with full duplex and 100 or better on the speed side. good luck.




Lui3
CCNP,CCDA,A+/Net+
Cisco Wireless Specialization

 

[ErrolDC]

There are no disadvantages to hard fixing a port to 100 full. As a matter of fact, we do it on all ports attached to servers. This eliminates giants and runts on our network. I've always had issues when we allowed auto-neg.

 

[lui3]

Giants and runts on the network as a result of auto negotiation? i have never heard of that? I have seen them caused from negotiation mismatches but never from autonegotiation itself. plus you should not see giants from one port appear on another port. they are physical layer errors and are terminated at the port itself. are you saying that you are getting giants/runts as a result of collisions caused by negotiation mismatches resulting in one port going 100 full and the other going 100 half?

just curious

Lui3
CCNP,CCDA,A+/Net+
Cisco Wireless Specialization

 

[juniperr]

Unix servers are natorious for needing a static speed and duplex set staticly or it will come up 10 half or mismatch negotiation. Novell has had similar issues where if left in auto on both sides you will accumulate runts and giants Im not sure why these two OS's do this it is documented on novell's site to set both static even if they say they are running the same speed ( I have seen a GWIA on one server with auto negotiate set connected to the same switch as the post office database server that had its ports set to static 100 full not be able to recieve mail but could send mail even though the speed and duplex said 100 full from negotiating, setting it static fixed the problem This was the actual fix recommended in a TID from novell and worked)something else is not negotiating correctly this only happens about 50% of the time on novell though I never really looked into why? maybe the IP stack patch level or something? I have never seen microsoft not auto negotiat correctly and is the only server OS I do not set static. I have worked on literaly thousands of servers and have seen this time and time again.

I always follow the rule Unix static, Novell static, Microsoft auto (unless Fastetherchannel). always set the switch and host static never let one auto while the other is static. full duplex is faster and no collisions between the host and switch so if at all possible run full duplex unless you are pushing ethernet 100 meter limit you may slow it down to 10 half and get more length from the run as 10baset is not as picky.

 

[CHM1]

When mode on the swith lights duples, you can see with the leds on the ports if you are full or half duplex on the ports.
For a port You must have duplex full or duplex half configured in a sh run command.
To come back to autonegociation duplex enter a no duplex half or no duplex full acordingly.
I did not experience problems with servers (all autonegociating), but with some PCs i had the following issues :
- impossible to work with some old 3COM NICs (either auto, full or half !) : solution : replace the NIC !
- impossible to work with some HP Vectra on a C2950 (the pCs worked fine with a C3524) : solution : upgrading the NIC driver on PCs (ca 50 PCs : good job !!)

 

[SCLDTony]

Thank you to everyone for your valuable input. I have just set all the ports to auto-negotiate and everything seems to be working fine. Thanks again!

Tony

 

[lui3]

great

Lui3
CCNP,CCDA,A+/Net+
Cisco Wireless Specialization